Tech Slot

Reported by G4DDK

The DX'er 50, part 3

Six metre transverter designs, in particular, are all about filtering. The DX'er 50 design uses lots of filtering as already described. However, I felt that the transmit low pass filter would be very critical to the success of the design. After looking at several published filters I decided to design my own.

This elliptical LPF design provides a 68dB deep notch between 100 and 104MHz, reducing to -60dB at third harmonic and higher. The measured insertion loss is less than 0.5dB at 50 to 52MHz, with a return loss of better than 20dB. The attenuation figure was improved upon slightly when I tried adding a fourth section to the filter, but the measured insertion loss increased noticeably. With the specified harmonic suppression of the output amplifier module (-25dB), adequate suppression of the critical second harmonic of over 90dB can be readily achieved with the three stages.

With 50mW input, the Mitsubishi M57735 will produce over 15Watt output across the entire 50 to 52MHz band. This is the amplifier I chose to use (because I have one, and I couldn't be bothered to design and build a discrete amplifier!).

It is important to provide adequate decoupling to the pins of the module to ensure stable operation. Equally important is the bias supply. Although the module takes less than 1Amp base bias, a 78S09 2 Amp regulator is recommended to ensure peaks of drive can be accommodated without small but potentially "unfriendly" bias potential changes. If you are really stuck in obtaining a 78S09, use a 7809, but keep output below 15Watts. Probably you ought to do that anyway, whichever regulator you use.

Collector supply to the module is not switched between transmit and receive, only the bias supply is switched. No broadband noise has been detected as a result of leaving the collector supply on during receive, and it does save having to switch a 5 Amp supply line. If this bothers you, turn it off with a relay during receive.

The transmit switching uses a solid state transistor arrangement that I have used in my transverters for many years. It is simple and reliable. I originally saw the circuit used in the OE9PMJ 23cm transverter and have used it extensively since.

It uses earth to transmit (ETT) input to turn TR1 on, which in turn turns TR2 off. On receive, TR2 is turned on by the 1K resistor between TR1 collector and ground. When ETT is applied via the isolating diode, D1, TR1 turns on, in turn pulling TR2 base towards the positive rail, turning it off. Both transistors have their emitters connected to the +ve suply rail. The receive line is connected to the collector of TR2 and the transmitter to the collector of TR1.

A red LED can be connected between TR1 collector through a 1k resistor to ground to indicate transmit, whilst a second (green?) LED can be connected between TR2 collector through a 1K resistor to ground to indicate receive on.

In the prototype, two small 12 volt operating relays are connected across the transmit supply to switch the mixer and filter between transmit and receive.

A Tohtsu CX120 coaxial relay switches the antenna between transmit and receive.

My transverter is housed in a diecast aluminium box 187mm wide, 118mm deep and 55mm high.

That completes the description of the DX'er 50. If you do build a copy of the transverter I would be pleased to hear how you got on and of any useful modifications you made.

Kits for the Mk 2 version of the Dx'er 50 (DX50) are available from Hands Electronics

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73 de Sam